Comment by amomchilov

Promotional to the number of _edges_. Edges are proportional to the square of the number of nodes, so the value of the network overall is proportional to the square of the nodes.

Think of it this way, for every new user added to the network: * the new user is enriched proportional to the number of existing users * every existing user is enriched by the 1 new user

This double-counting is what gives it the quadratic growth.